The invention is particularly suitable for roller mills for grinding cement clinker and granulated blast furnace slag for the building material industry. The roller mills usually comprise a classifier which is generally integrated into the roller mill but can also be arranged outside of the roller mill.
EP 0 406 644 B1 discloses an air swept roller mill of the LOESCHE type, wherein the comminution of cement clinker or granulated blast furnace slag is carried out through a combination of grinding rollers and preparation rollers. The preparation rollers serve for the compression and smoothing of the grinding bed. A preparation roller is assigned to each grinding roller and forms with this a pair of rollers. Air swept roller mills with two or three such roller pairs are known which are arranged symmetrically and at the same distance from each other and roll on a rotating grinding pan or a grinding bed formed thereon. A preparation roller is thereby arranged before the respectively assigned grinding roller in order to roll over the grinding material which is fed centrally onto the rotating grinding pan and moves under the effect of the centrifugal force to a grinding track on the pan edge. This grinding material consists of a mixture of coarser particles of the freshly supplied material and smaller particles not yet comminuted to the product fineness which are rejected by the classifier arranged over the grinding chamber and fed back to the centre of the grinding pan.
FIG. 1 shows the operating principle of a known roller pair. A grinding roller 24 and a preparation roller 26 roll on a grinding bed 25 formed by the grinding material to be comminuted. Arrow A indicates the movement direction of the grinding pan or tray 23 with the grinding track 22. Arrow B is intended to illustrate the material feed and arrows C indicate the rotation direction of the grinding roller 24 and preparation roller 26 rolling on the grinding bed 25. After the supply of the material according to arrow B there is a greatly aerated grinding bed 25.1, over which the preparation roller 26 now rolls. Only a pre-compression and compaction of the material thereby takes place with the formation of a compressed grinding bed 25 from which air has been removed. Comminution does not usually take place due to the low forces under the preparation roller 26. It is only once in the grinding gap between the subsequently arranged grinding roller 24 and the horizontal grinding track 22 that the desired comminution takes place. The comminuted grinding material 25.2 then passes due to the centrifugal forces over the pan edge and is conveyed with the aid of a gas flow pneumatically to the classifier above the grinding chamber (not shown).
The grinding rollers 24 and the smaller preparation rollers 26 are conically formed and are arranged in the region of the circular grinding pan 23 with grinding track 22 in such a way that their running surfaces roll as closely as possible to the outer grinding track edge 27 or pan edge and thus have a virtually identical pitch circle radius or running ring. FIG. 2 illustrates the preparation roller 26 of this known arrangement.
In the air swept roller mill known from DE 42 02 784 C2 with two roller pairs each comprising a grinding roller and a preparation roller or pre-compression roller the speed of the preparation rollers can be regulated progressively in order to reduce the vibrations of the air swept roller mill caused by the grinding process to a minimum. Grinding rollers and preparation rollers roll with their running surfaces close to a retention rim which is arranged going around the outer edge of the grinding pan and defines the outer side of the grinding track.
DE 44 42 099 C2 and EP 0 792 191 B1 disclose a roller mill, wherein the pre-compression or preparation rollers are designed as retention rollers in order to influence the movement of grinding material between the grinding rollers and form a weir-like retaining device or a rotating retention wall. Adequate grinding material supply of the grinding rollers is thereby to be ensured at normal and also increased grinding material speed and a higher throughput with relatively low resources is thereby to be guaranteed.
The retention rollers are arranged close to each other and adjacent to the grinding rollers and roll like the grinding rollers on a running ring close to the grinding pan edge. Levelling or pre-compression of the grinding bed takes place with shell surface regions of the retention rollers which are formed between the retention areas and retention rings.
Trials and computer-assisted DEM simulations as well as evaluations of image material of experimental trials have shown that the known arrangement of the preparation rollers on the pan outer edge means that they do not completely roll over the grinding material flowing from the centre to the grinding rollers and a pre-compression does not take place in the necessary way. A part flow of the grinding material which is moved spirally from the centre of the grinding pan to the outer edge or to the grinding track moves inwards past the end face of the preparation rollers and thus arrives without pre-compression at the subsequent grinding roller. It has also been ascertained that a part flow of the grinding material already pre-compressed by a preparation roller is thrown over the pan edge under the effect of the centrifugal force before the subsequent grinding roller and is thus no longer comminuted by the subsequent grinding roller. This flow of grinding material which leaves the grinding pan respectively between a preparation and grinding roller increases the inner material circulation in the grinding and classifying chamber and necessitates additional energy for the pneumatic transport to the classifier. As this part flow has not yet been ground to product fineness it is rejected in the classifier and falls via a middlings cone back into the pan centre and reaches the grinding track. From an energy viewpoint this means that not only additional energy must be expended for the pneumatic transport but also the energy from the grinding pan drive for the pre-compression is lost.